A theoretical analysis of an ultra-short pulse converted to Terahertz radiation via optical rectification in nonlinear optical crystal is presented here; several factors that affect optical-to-THz conversion efficiencies are discussed; pulse durations affect the conversion efficiency effectively: when crystal length is equal to the optimal crystal length lc, optical-to-THz conversion efficiency is the highest, but for the periodically-inverted electro-optic crystals, conversion efficiency is almost proportional to the crystal length when absorption can be neglected. Taking account of the absorption of crystals, effective length of crystal is Leff=0.63/α, there is no apparent increase of conversion efficiency and the conversion efficiency approaches to a constant eventually when the crystal length is increased.
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